This invention relates to variable phase shifters and, more particularly, to an arrangement for shifting phase while maintaining a constant amplitude output independent of the phase setting.
There are numerous applications for phase shifting circuits in the fields of communications and measurements. One such application is described as a phase resolver in U.S. Pat. No. 4,157,508 issued to R. P. Hecken on June 5, 1979. The terminology of phase resolver is used since two output signal components are produced with a controlled phase relationship between them. The basic purpose of the arrangement in the Hecken patent is to provide predistortion compensation, a well-known technique in the art, for canceling the distortion introduced by the nonlinear transfer characteristics of electromagnetic devices. This technique requires precise phase control of the predistortion component to effect destructive phase interference during the signal amplification process.
The problem with conventional phase resolvers is that phase and amplitude controls are interactive. Phase control is provided by two adjustments which determine the relative magnitude of two phase displaced signals. Unfortunately when these two signals are vectorially combined, the amplitude of the output signal varies with phase setting. Therefore, a third interactive adjustment is also required in order to control the signal amplitude. Thus, setting up a predistorter and maintaining its performance level as the electromagnetic device ages is a tedious problem since the interactive property of conventional controls requires numerous repetitive resettings of controls in the process of obtaining acceptable performance from a predistorter.